During an oil-water separation process, insoluble surfactants and solids accumulate in the separator between the oil and water phases and at the bottom of the water phase. The accumulation of these components is caused by emulsions and sludges equilibrating at different levels in the oil-water separator vessel based on their bulk density. The continued accumulation of these materials can ultimately result in hydrocarbon entrainment in the aqueous effluent exiting the bottom of the vessel and also limit the ability to adequately control the level of the oil-water interphase in the vessel.
Emulsion breaking chemicals are often added ahead of the separator to aid in de-oiling the solid particles so that they can effectively migrate from the oil phase or from the emulsion interphase into the water phase. These solid particles typically include sand, silt, drilling mud, iron oxide, iron sulfide, and other naturally occurring and foreign contaminants. Heavy insoluble asphaltenic hydrocarbons can also fall out of a crude oil in a separation vessel. This de-oiling, however, is typically not complete.
The more de-oiled portion of these materials, commonly referred to as solids, stays suspended in the water phase and are thus removed from the separator vessel with the aqueous discharge. The less de-oiled, but heavier than water, portion falls to the bottom of the vessel and loosely agglomerates there, being bound by the heavy oils remaining on the solid surfaces. Over time, this solid layer will compact on the bottom of the vessel, making it increasingly difficult to remove.
Refinery desalter systems are designed to remove primarily inorganic salts from a crude oil prior to refining. The desalting step is provided by adding and mixing with the crude a few volume percentages of fresh water to contact the brine and salts present in the crude. The resulting oil-water emulsion need necessarily be separated for further processing of the crude oil. This separation is typically carried out at temperatures from 225.degree. F. to 325.degree. F. and pressures from 100 psi to 200 psi.
Many desalter systems are equipped with mud wash piping designed to periodically agitate the water on the bottom of the desalter and remove the accumulation of oily solids. Water is pumped into the mud wash header at rates of two to ten gallons per minute, per nozzle, exiting the nozzle as a turbulent jet directed at the bottom of the vessel.
Mud wash systems are of two basic types. The once through system uses water that is diverted from the incoming desalter wash water, whereas the recirculating system uses effluent brine diverted from the desalter. Both systems perform essentially the same function.
Mud washing is performed for varying durations and frequencies depending on historic operating practice and feedstock contaminant levels. Typical duration is from ten minutes to eight hours and typical frequency is from once every four hours to once per week.
It is also typical to set the duration based on the visual appearance of the effluent brine during mud washing. In this situation, the effluent brine is visually clear prior to mud washing. The mud wash will be started, thereby discoloring the water with solids and oil from the bottom of the desalter (the "mud") and from any emulsion that is disengaged from the oil-water desalter interphase. The mud wash will be continued until the solids and oil are removed to the extent that the water is clear again, at which point the mud wash is discontinued.
Due to turbulence created in the desalter during the mud washing, it is common to entrain some oil and emulsion from the interphase into the desalter brine. This is especially true when solids have accumulated in the bottom of the desalter reducing available water volume and creating uneven water distribution. In addition, the mud itself can contain significant oil attached to solids which makes disposal of the mud a more costly and time consuming proposition.